That would all be my fault…
The config page has a minor glitch. If you have an empty admin_pw it just won’t show. This is fixed in r1084, but for now locate your mt-daapd.conf and set a password:
admin_pw = p
I forgot to include the spinner.gif in the subversion repo. It was added in r1083, it is only used on the experimental plalist.html page.
The double slash is just a log issue, if the file is present it will be fetched all right. The request is GET /spinner.gif HTTP/1.1 blabla and when it is missing it is displayed as //spinner in the log.
edit: found the //spinder.gif problem